The present invention relates to an anti-skid control system executing anti-skid control (simply, skid control) according to which brake pressure at a wheel-brake cylinder is controlled to prevent wheel lock-up during braking, and particularly to an anti-skid control system capable of controlling the wheel-brake cylinder pressure by way of on/off reaction of solenoid valves incorporated within a hydraulic modulator.
As is generally known, an anti-skid control system, usually abbreviated to an xe2x80x9cABSxe2x80x9d system, is used many automotive vehicles, for controlling the wheel-brake cylinder pressure in such a manner as to prevent wheel lock-up during braking, thus ensuring a stable vehicle behavior (enhanced vehicle stability). Such an anti-skid control system is generally constructed to properly repeatedly execute a pressure build-up control mode at which the wheel-brake cylinder pressure is built up, a pressure reduction control mode at which the wheel-brake cylinder pressure is reduced, a pressure hold control mode at which the wheel-brake cylinder pressure is held constant, and a moderate pressure build-up control mode at which the wheel-brake cylinder pressure is gradually built up, depending upon the relative relationship between the vehicle speed and the wheel speed, that is, a slip ratio. During the wheel-brake cylinder pressure control (during skid control), the operation of an electronically controlled fluid-pressure control valve, such as a pressure build-up valve or a pressure reduction valve, is controlled by means of an ABS control module or an ABS control unit. In an anti-lock control device disclosed in Japanese Patent Provisional Publication No. 6-144195, for the purpose of skid control, PID (proportional plus integral plus derivative) control is made to a linear-actuated solenoid valve capable of continuously controlling a brake-fluid pressure by adjusting a value of current applied thereto. A combination of the PID control and the linear-actuated solenoid valve ensures a high control accuracy. However, the linear-actuated solenoid valve is so expensive and complicated in construction. Also, the derivative component of the PID control is apt to be affected by noises input from the road surface, and thus there is a possibility of excessive pressure build-up or excessive pressure reduction. In contrast, in the brake hydraulic pressure controller disclosed in Japanese Patent Provisional Publication No. 7-117653, on/off reaction of solenoid valves incorporated within a hydraulic modulator is controlled by way of duty-cycle control. In such an ABS system employing duty-cycle controlled solenoid valves within a hydraulic modulator, taking into account road surface conditions, such as a so-called xcexc-jump road surface condition that a transition between a low-xcexc road and a high-xcexc road occurs, a moderate pressure build-up operating mode or a moderate pressure reduction operating mode is executed as follows.
For instance, in order to achieve the moderate pressure build-up, the pulse width of pulse signals output to the solenoid valve during a preset period of time (or a predetermined constant operating cycle) is controlled to gradually increase from 3 msec through 4 msec and 5 msec to 6 msec, in that order. In contrast, assuming that the pulse width of pulse signals output during the pressure build-up operating mode is kept constant and additionally the pressure build-up rate is set at a relatively high value, there is a tendency for the wheel to lock up again during the initial pressure build-up operating mode executed after the pressure reduction operating mode, during driving on a low-xcexc road surface. Assuming that the pressure build-up rate is set at a relatively low value with the pulse width kept constant, there is a problem of an undesirably long pressure build-up time interval and a lack of the braking force. Furthermore, in the event that a so-called xcexc-jump or xcexc-change that a friction factor of the road surface changes from low to high takes place with the relatively low pressure build-up rate and with the pulse width kept constant, a recovery time to an appropriate wheel-brake cylinder pressure suitable for the high-xcexc road surface condition tends to become undesirably long. This also causes a lack of the braking force.
In contrast to the above, when gradually increasing the pulse width for the preset time period so as to gradually increase the pressure build-up rate with the lapse of the pressure build-up time, it is possible to properly build up the wheel-brake cylinder pressure, preventing wheel lock-up from occurring again during driving on the low-xcexc road. In the event that a so-called xcexc-jump occurs, it is possible to effectively reduce the recovery time to the appropriate wheel-brake cylinder pressure suitable for the high-xcexc road surface condition, thus avoiding the lack of braking force.
In the same manner as discussed above, assuming that the pulse width is kept constant and additionally the pressure reduction rate is kept at a relatively high value, there is a tendency for brake fluid in the wheel-brake cylinder to be excessively returned via a reservoir to the master-cylinder side by means of an ABS pump (a return pump). This increases the recovery time to an appropriate wheel-brake cylinder pressure at the next pressure build-up operating mode. Additionally, the amount of brake fluid being pumped out by the ABS pump tends to increase. Conversely when the pressure reduction rate is kept at a relatively low value, it takes a long time until the wheel lock-up condition has been neutralized. In particular, when a xcexc-jump that a friction factor of the road surface changes from high to low occurs, a recovery time to an appropriate wheel-brake cylinder pressure suitable for the low-xcexc road surface condition tends to become undesirably long. To avoid this, during the moderate pressure reduction operating mode, an ON pulse width of pulse signals output to the pressure reduction solenoid valve is gradually increased with the lapse of the pressure reduction time. However, the previously noted conventional anti-skid control system employing the duty-cycle controlled solenoid valves has the following drawbacks.
That is, when gradually increasing the pulse width in accordance with an increase in the pressure build-up time or when gradually increasing the pulse width in accordance with an increase in the pressure reduction time, at the last stage of the predetermined operating cycle, there is a tendency for the wheel-brake cylinder pressure to overshoot the desired wheel-brake cylinder pressure level. Owing to the overshoot, brake fluid is wastefully exhausted from the wheel-brake cylinder to the reservoir. This increases an amount of work of the ABS pump serving to induct the brake fluid in the reservoir into the master cylinder side. That is to say, in the conventional ABS system, there is a limitation to an adaptability to a rapid change in the friction factor of the road surface, such as in the event of the occurrence of the xcexc-jump road condition. The above-mentioned overshooting deteriorates the convergence to the desired pressure level. Additionally, owing to the occurrence of overshooting, a large-capacity of ABS pump is required. This increases the ABS system manufacturing costs and also results in an increased weight of the ABS system. When gradually increasing the ON pulse width during the predetermined constant operating cycle, there is a problem of a resonance occurring due to the predetermined constant operating cycle of the ABS system. There is another problem of a great change in the wheel-brake cylinder pressure, occurring in the event of the output of a pulse signal having a wide ON pulse width. This deteriorates the noise/vibration reduction performance.
Accordingly, it is an object of the invention to provide an anti-skid control system capable of controlling a wheel-brake cylinder pressure by on/off reaction of solenoid valves, which system effectively enhances a noise/vibration reduction performance.
In order to accomplish the aforementioned and other objects of the present invention, an anti-skid control system for an automotive vehicle comprises a wheel speed sensor that outputs a sensor signal indicative of a wheel speed at each of road wheels on the vehicle, a hydraulic modulator having electromagnetic solenoid valves for regulating a wheel-brake cylinder pressure at each of the road wheels, a skid control unit configured to be electronically connected to the wheel speed sensor and the solenoid valves for preventing a wheel lock-up condition by controlling the wheel-brake cylinder pressure via on/off reaction of the solenoid valves in response to a pulse signal based on the wheel speed, the skid control unit comprising a desired wheel speed generation section that generates a desired wheel speed based on the wheel speed, a wheel-speed deviation calculation section that calculates a wheel-speed deviation between the desired wheel speed and the wheel speed, a desired brake-fluid pressure calculation section that calculates a desired brake-fluid pressure based on an integrated value of the wheel-speed deviation, a controlled ON pulse width setting section that sets a controlled ON pulse width based on the desired brake-fluid pressure, and a pulse output control section that outputs the pulse signal having the controlled pulse width to the solenoid valves after a lapse of a preset pulse width from a previous pulse signal output.
According to another aspect of the invention, an anti-skid control system for an automotive vehicle comprises a wheel speed detection means for outputting a sensor signal indicative of a wheel speed at each of road wheels on the vehicle, hydraulic modulating means having electromagnetic solenoid valves for regulating a wheel-brake cylinder pressure at each of the road wheels, skid control means configured to be electronically connected to the wheel speed detection means and the solenoid valves, for preventing a wheel lock-up condition by controlling the wheel-brake cylinder pressure via on/off reaction of the solenoid valves in response to a pulse signal based on the wheel speed; the skid control means comprising desired wheel speed generation means for generating a desired wheel speed based on the wheel speed wheel-speed deviation calculation means for calculating a wheel-speed deviation between the desired wheel speed and the wheel speed, desired brake-fluid pressure calculation means for calculating a desired brake-fluid pressure based on an integrated value of the wheel-speed deviation, controlled ON pulse width setting means for setting a controlled ON pulse width based on the desired brake-fluid pressure, and pulse output control means for outputting the pulse signal having the controlled pulse width to the solenoid valves after a lapse of a preset pulse width from a previous pulse signal output.
According to a further aspect of the invention, an anti-skid control method of an automotive vehicle employing an anti-skid control system for preventing a wheel lock-up condition by controlling a wheel-brake cylinder pressure at each of road wheels via on/off reaction of solenoid valves of a hydraulic modulator in response to a pulse signal based on the wheel speed, the method comprises detecting a wheel speed at each of the road wheels on the vehicle, generating a desired wheel speed based on the wheel speed, calculating a wheel-speed deviation between the desired wheel speed and the wheel speed, calculating a desired brake-fluid pressure based on an integrated value of the wheel-speed deviation, setting a controlled ON pulse width based on the desired brake-fluid pressure, and outputting the pulse signal having the controlled pulse width to the solenoid valves after a lapse of a preset pulse width from a previous pulse signal output.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.